This invention relates to brake testing systems, and particularly to such systems for testing the adequacy of the brakes of automobiles and similar two-axle vehicles.
Faulty or inadequate brakes are a significant cause of automobile accidents. Moreover, the faulty condition of the brakes is often not apparent during routine use of the brakes. The operator of the vehicle is usually not aware, until it is too late, that the vehicle's brakes are deficient.
Furthermore, many brake defects are not readily apparent during a visual inspection of the brakes. Only a test simulating actual shopping conditions can detect many brake defects.
When the brakes of a vehicle are applied, a retarding force is generated between the tire and the surface on which the tire is riding. When this force becomes greater than the weight on that wheel multiplied by the coefficient of friction between the tire and the surface, the wheel will begin to lock up and stop rolling. The retarding force of a wheel just before lockup is greater than the retarding force of the same wheel just after lockup. In addition, a locked wheel loses its ability to maintain lateral forces, which makes handling very difficult--especially when the rear wheels lockup.
Since the maximum usable force of a brake is related to the weight on that wheel, vehicle designers adjust the braking system so that the brake force distributions coincide with the vehicle weight distribution. Since the left side of the car weighs very close to the right side of the car, there is rarely a difference in design between brakes on the same axle. However, since most passenger cars weigh more on the front axle than on the rear axle, front brakes are almost always designed to handle more brake force than rear brakes.
When a vehicle is decelerating, the forces acting on the center of gravity of the vehicle cause a weight shift. The front axle weight is increased, and the rear axle weight is decreased. This increases the chance of rear axle lockup since the same brake force is now retarding a smaller tire-to-road force. This situation is dangerous since locked tires cannot hold their lateral forces. The rear axle will begin to "fishtail" and eventually the vehicle will go into a spin. For this reason, brake systems include a proportioning valve that senses high brake pressures and reduce hydraulic pressure to the rear axle brakes during hard stops.
The ideal vehicle is designed to be "neutral balance," which means that no one wheel will lock up prematurely. If the rear axle locks up first, a vehicle is said to be "rear biased." If the front axle locks up first, the vehicle is "front biased." Many vehicle manufacturers design slightly front biased vehicles in order to avoid the dangerous rear lockup situation.
Apparatus for testing brake performance are available, but they could be improved. For example, the results of the brake test under actual stopping conditions can depend upon the make and model of the vehicle, the actual deceleration applied to the vehicle during the test, and the static and dynamic weight distribution of the vehicle during the test. Apparatus which would take into account all these factors have heretofore been considered too complicated or too slow. For example, a system which would take into account the make and model of the actual vehicle under test would appear to require that the operator enter that information into the system. Entering this information, however, unacceptably increases the total time required for performing what is otherwise a short (thirty second) test.
Present brake testing systems compensate for the lack of information about the specific vehicle under test and the dynamic weight distribution under test by setting broader than necessary ranges for the acceptability of the measured braking forces. In some cases these broad ranges allow vehicle brakes to "pass" which, for that particular vehicle and deceleration, should have failed.
In order to expedite testing the vehicle's brakes, it is also desirable to display to the operator the test results while the operator is still in the vehicle. Some present brake testers accomplish this with a pair of indicator lights, one for "pass" and the other for "fail." This, of course, is not particularly satisfying for the consumer who is faced with the prospect of unanticipated brake work. A display which clearly indicates the precise nature of the defect would be desirable.
In addition, for a test to be valid, it is necessary that a certain minimum deceleration be applied to the vehicle. Present brake testers do not immediately and clearly indicate to the operator, while he is still in the vehicle, that the deceleration was too low. If the operator could tell at a glance that the deceleration was too low, he could simply redo the test while seated in the vehicle.